Classically, zeolites are defined as aluminosilicates with open 3-dimensional framework structures composed of corner-sharing TO4 tetrahedra, where T is aluminium or silicon. Cations that balance the charge of the anionic framework are loosely associated with the framework oxygens and the remaining pore colume is filled with water molecules. The non-framework cations are generally exchangeable and the water molecules removable. The tetrahedra are connected forming rings and cages including pores and pore networks that extend throughout the zeolite crystal. The rings of a zeolite are usually formed of 8, 10, or 12 tetrahedral units, although zeolites having larger rings have been synthesised. The framework type of a zeolite is assigned a three letter framework type code by the International Zeolite Association (IZA). For example, MOR is assigned to zeolitic materials which have the framework structure of mordenite. Descriptions of all the framework types that have been assigned codes by IZA are included in the Database of Zeolite Structures (www.iza-structure.org). Zeolite structures are also defined in the ‘Atlas of Zeolite Structure Types’, W M Meier, D H Olson and Ch Baerlocher, 6th revised edition, 2007, Elsevier.
Zeolites having framework type code MOR have 12-membered ring main channels with intersecting 8-membered ring channels (side-pockets). Mordenites have framework type code MOR.
Zeolites, in general, both natural and synthetic, have been demonstrated to have catalytic properties for various types of chemical processes. In particular, mordenites have been shown to catalyse the carbonylation of methanol and/or dimethyl ether with carbon monoxide to produce acetic acid and/or methyl acetate. For example, the carbonylation of methanol with carbon monoxide in the presence of a copper, nickel, iridium, rhodium or cobalt loaded mordenite catalyst to produce acetic acid, is described, for example in EP-A-0 596 632. In WO 2006/121778 there is described a process for the production of a lower alkyl ester of a lower aliphatic carboxylic acid by carbonylating under substantially anhydrous conditions a lower alkyl ether with carbon monoxide in the presence of a mordenite or ferrierite zeolite catalyst. In WO 2005/105720 there is described a process for the preparation of carboxylic acids and derivatives thereof in the presence of mordenite which has framework elements in addition to aluminium and silicon and which has also been loaded with copper, nickel, iridium, rhodium or cobalt.
The article ‘Specificity of Sites within Eight-Membered Ring Zeolite Channels for Carbonylation of Methyls to Acetyls’ Bhan A et al, J. Am. Chem. Soc. 2007, 129, 4919-4924, discusses the reactivity of CH3 groups located within eight-membered ring channels in mordenite and ferrierite in carbonylation reactions.
U.S. Pat. No. 3,551,353 describes a process for the dealumination of mordenite by contacting steam and mineral acid in alternate steps. The dealuminised mordenite is disclosed to be active for hydrocarbon conversion reactions such as cracking.
U.S. Pat. No. 5,238,677 describes a process for the dealumination of a zeolite having the structure of mordenite by contacting the zeolite with a dicarboxylic acid and steaming.
U.S. Pat. No. 4,654,316 describes a process for selective surface dealumination of zeolites by sequential ion-exchange and calcination to improve catalyst selectivity in hydrocarbon conversion reactions.
It is highly desirable in carbonylation processes to improve catalyst performance. Performance measures include product selectivity, product yield, catalyst stability and lifetime.
In carbonylation processes which use mordenite as catalyst, the target products are acetic acid and/or methyl acetate. The production of hydrocarbons in these carbonylation processes is highly undesirable. Hydrocarbons can form a layer of coke on the catalyst which leads to catalyst deactivation. Without wishing to be bound by theory, it is believed that, in carbonylation processes, the reactions leading to the formation of hydrocarbon by-products take place in the 12-membered ring channels of a MOR zeolite whilst the reactions leading to the formation of carbonylation products is believed to take place in the 8-membered ring channels. Thus, it would be highly desirable to be able to preferentially remove aluminium from the 12-membered ring channels of a MOR zeolite, thereby leading to a catalyst having improved performance in carbonylation processes.